Dimensional checkings performed by contacting mechanical parts--by means of heads having feelers contacting the surface of the part--involve sliding motions between the feeler and the surface for arranging the feeler in correspondence with a portion to be checked. In particular, these checkings can be carried out in dynamic way , i.e. the position of the movable feeler can be measured during said sliding motions, that may be manually or automatically controlled. For example, in connection with dynamic checkings it may be foreseen to detect, by means of mechanical or electric datums, the passage of the feeler along a predetermined portion to be checked, or to locate this portion depending on the output signals of the head during the sliding motions.
A simple manual checking of this type is carried out by a plug gauge, for example like that described in the U.S. Pat. No. 4 884 346, that is inserted into a hole, the diameter of which must be checked, and advanced up to a depth defined by a flange. During the advance two feelers slide on the hole surface, resiliently urged by a spring and by the structure of the arm-set, that also comprises transducer means.
A similar device, in which the transducer means are obtained by strain gages, is described in international application WO-A-89/03507, and can be used for checkings of automatic type, according to a method corresponding to the prior art portion of claim 1, in which the plug gauge is arranged on a slide and is displaced by it from a rest position to a measurement position.
In addition to plug gauges for internal dimensions like those mentioned above, there are many known devices and instruments with gauging heads of contacting type featuring sliding motions between feelers and surfaces. As an example only, it is possible to mention snap gauges for external diameters used in apparatuses like that described in international application WO-A-89/04455, featuring feelers that are displaced into contact with cylindrical external surfaces and caused to slide on said surfaces towards portions to be checked, located by mechanical datums.
In each of the above mentioned cases, the positioning of the feeler (or the feelers) on a surface of the part takes place through a sliding motion of the feeler on the surface along a determined direction or path. It is evident that the friction arising through said sliding motion, although limited in view of the particular shape and material of the feeler, cannot be eliminated. The resulting strains transmitted to the elements of the gauging heads can cause malfunctions due to flexures in the movable arm-set and/or strains in the transducer elements, for example when the latter are obtained by strain gages. Principally, repeatability errors of the heads can occur, due also to differences in the surface finishing of the different parts, said errors being difficult or impossible to evaluate.
The influence of these errors can be particularly high in high precision apparatuses that comprise very sensitive gauging heads, with arm-sets having small dimensions and low weight.